Many happy returns to the three children of Norway's Henriksen family – Heidi (b. 1960), Olav (b. 1964) and Lief-Martin (b. 1968). What's so unusual about this? They were all born on Leap Day, February 29, the most siblings to do so.

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Leap days are necessary because there are not precisely 365 days in a year. The exact figure is 365.2425 days, which means that we gain an extra day every four years.

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During the reign of Emperor Julius Caesar, when there were only 355 days in a year, the seasons would get out of sync with the calendar. Caesar decreed that the calendar be revised, a new month be added in his name (July), and an extra day added every
four years to keep the calendar and seasons together. The date of February 29th was chosen as the 28th was then considered the last day of the year.

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Most Generations Born on Leap Day

The only verified example of a family producing three consecutive generations born on February 29 (Leap Day) is that of the Keogh's. Peter Anthony (Ireland, b.1940), his son Peter Eric (UK, b. 1964) and his grand-daughter Bethany Wealth (UK, b. 1996) all celebrate their birthdays infrequently (every four years).

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There are a total of 1,461 days are in each four-year period, so the probability that a person is born on February 29 is 1 in 1,461. But, it's not quite this simple.

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If a year is divisible by 100 then it is a Leap Year only if it is also divisible by 400, so between the years 2097 and 2103, your chance of being born on Leap Day is zero.

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The Leap Day was introduced in 46 BC, but it was discovered in 10 BC that the priests responsible for maintaining the calendar had added too many – once every three years, instead of the four decreed by Julius Caesar. To compensate, there were no more
leap days until AD 8.

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Since then, there have been Leap Years every year divisible by 4, except for those years that can be divided by 100 but not 400.